Cellulose filaments (CF) previously referred to as cellulose nanofilaments (CNF) are known to have many interesting properties one of which is increasing the dry and wet strength properties of paper when used as an additive in the production thereof. They are produced by refining wood or plant fibers at a high to very high level of specific energy using high consistency refiners (Hua, X., et al. High Aspect Ratio Cellulose Nanofilaments and Method for their Production. PCT/CA2012/000060; WO 2012/097446 A1, 2012). They have superior reinforcement ability over cellulose microfibrils or nanofibrils such as microfibrillated cellulose (MFC) or nanofibrillated cellulose (NFC) prepared using other methods for the mechanical fibrillation of wood pulp fibers, because of their longer lengths and higher aspect ratio as a result of their unique production process which minimizes fiber cutting.
The production of cellulose filaments occurs in suspension with water at a consistency up to 60%. Consistency is the weight percentage of a cellulose material in a mixture of the cellulose material and water. One serious drawback to using cellulose filaments is the difficulty of preparing dry cellulose filaments without decreasing their dispersibility in aqueous media and/or their reinforcement ability. This difficulty is similar to that for the drying of other cellulose microfibrils or nanofibrils or even pulp fibers by conventional means, and is due to so-called hornification. Hornification is attributed to many factors that include: the formation of irreversible hydrogen bonds (H-bonds) and/or the formation of lactone bridges (Fernandes Diniz, et al., “Hornification—its origin and interpretation in wood pulps,” Wood Sci Technol, Vol. 37, 2004, pp. 489-494). Hornification produces a dried cellulose filament material that cannot be re-dispersed into water, a water solution or a water suspension, such as a pulp and paper suspension, when the dry cellulose filaments are mixed with wood pulps in a pulper or mixing chess for usage as a paper strengthening additive.
To avoid the disadvantage of irreversible hornification that produces non-dispersible microfibrillated cellulose (MFC) or nanofibrillated cellulose (NFC), two approaches have been attempted: 1) processing MFC with additives or 2) derivatizing MFC or NFC.
Each of these approaches has its disadvantages. With the first approach to reducing hornification, MFC are dried with additives that block the formation of H-bonds and help to prevent H-bond or lactone bridge formation (Herrick, F. W., U.S. Pat. No. 4,481,076; Lowys, M.-P. et al, “Rheological Characterization of Cellulosic Microfibril Suspensions. Role of Polymeric Additives,” Food Hydrocolloids, Vol. 15, 2001, pp. 25-32; and Cantiani, R. et al. U.S. Pat. No. 6,306,207 B2). These additives include: sucrose, glycerin, ethylene glycol, dextrin or carboxymethyl cellulose. Here the main drawback is the large quantity of the additives required, in some cases more than 15% by weight are used.
The second approach to reducing hornification in MFC or NFC during drying is to derivatize the microfibrillated or nanofibrillated cellulose with the introduction of various groups including carboxyl groups (Eyholzer, C. et al, “Preparation and Characterization of Water-Redispersible Nanofibrillated Cellulose in Powder Form,” Cellulose, Vol. 17, No. 1, 2010, pp. 19-30; Cash, M. J. et al. Derivatized Microfibrillar Polysaccharide U.S. Pat. No. 6,602,994 B1). However, the derivatization requires the use of large amounts of the reagent, for example, 5.81 g of monochloroacetic acid (MCA) (7.26 g of 80% MCA) per 36 g of MFC in an isopropanol and water solution under a nitrogen atmosphere. It has not been established that MFC derivatized with MCA or other molecules can be re-dispersed in water after drying